For improving output power and reducing signal distortion, a power amplifier circuit is usually embodied as a multi-stage amplifier. For example, a plurality of amplifier circuits are connected in series for amplifying the voltage, current and power of the signal respectively.
Referring to FIG. 1, there is shown schematic diagram of a portion of a power amplifier circuit in accordance with the prior art. In a radio frequency power amplifier circuit, a matching circuit 14 is disposed between the amplifier circuits 12 and 16 for impedance matching and improving the power transmission and frequency response of a signal.
Since there are active elements in the amplifier circuits 12 and 16, the impedances of the amplifier circuits 12 and 16 may behave as a negative resistance under some conditions. When the input impedance of the amplifier circuit 16 at the node 15 behaves as a negative resistance and the output impedance of the amplifier circuit 12 at the node 15 conforms to certain conditions, the amplifier circuits 12 and 16 interact with each other and resonate. This causes the circuit to oscillate and degrades the stability of the circuit.
For preventing the circuit from oscillation, people usually connect a resistance in series between the amplifier circuit 16 and the node 15 to compensate the negative resistance at the node 15. Till the input impedance at the node 15 toward the amplifier circuit 16 does not behave as a negative resistance, the circuit would not oscillate and can keep stable.
Although the aforementioned method prevents the circuit from oscillation, the resistance on the signal path attenuates the signal and reduces the efficiency of the circuit.
Consequently, how to prevent the circuit from oscillation without reducing the efficiency of the circuit is the problem of the community.